IP
Instances

The first decision
you need to make when you use this API is whether to accommodate multiple
instances of IP running in the kernel or to only interact with the global
zone.

To be aware of the presence of IP instances, register callback
functions that are activated when an instance is created, destroyed, and shut
down. Use net_instance_alloc() to allocate a net_instance_t packet event structure to store these three function pointers.
Use net_instance_free() to free resources when you no longer
need the callbacks and the structure. Specify nin_name to
give the structure instance a name. Specify at least the nin_create() and nin_destroy() callbacks. The nin_create() function
is called when a new instance of IP is created, and the nin_destroy() function
is called when an instance of IP is destroyed.

Specifying nin_shutdown() is optional unless the code will be exporting information to kstats.
To use kstats on a per-instance basis, use net_kstat_create() during
the create callback. Cleanup of the kstat information must happen during the
shutdown callback, not the destroy callback. Use net_kstat_delete() to
clean up kstat information.

If one or more instances of IP are present when net_instance_alloc() is called, the create callback will be called
for each currently active instance. The framework that supports the callbacks
ensures that only one of the create, destroy, or shutdown functions is active
at any one time for a given instance. The framework also ensures that once
the create callback has been called, the shutdown callback will only be called
after create has completed. Similarly, the destroy callback does not start
until the shutdown callback is complete.

The mycreate() function
in the following example is a simple example of a create callback. The mycreate() function records the network instance identifier in its
own private context structure and registers a new callback to be called when
a new protocol (such as IPv4 or IPv6) is registered with this framework.

If no zones are running (and therefore no instances other than the global
zone), calling net_instance_register() runs the create
callback for the global zone. You must supply the destroy callback so that net_instance_unregister() can be called later. Attempts to call net_instance_register() with either the nin_create or nin_destroy fields set to NULL will fail.

The function mynewproto() should
expect to be called each time a network protocol is either added to or removed
from a networking instance. If registered network protocols are already operating
within the given instance, then the create callback will be called for each
protocol that already exists.